A variety of medical devices for delivering a therapy and/or monitoring a physiological condition have been used clinically or proposed for clinical use in patients. Examples include medical devices that deliver therapy to and/or monitor conditions associated with the heart, muscle, nerve, brain, stomach or other organs or tissue. Some therapies include the delivery of electrical stimulation to such organs or tissues. Others include the delivery of one or more drugs within or outside of a patient's body.
Some medical devices include one or more electrodes for the delivery of electrical stimulation to such organs or tissues and/or for sensing various indications of patient health. Some medical devices include electrodes to sense intrinsic electrical signals of the heart, and/or other sensors for sensing various physiological parameters of a patient. Some medical devices include sensors for sensing mechanical contractions of the heart.
In some cases, a medical device senses one or more indications of cardiac cycles, e.g. a rate of intrinsic depolarizations, repolarizations, and or contractions of a patient's heart. One or more heart rate signals may be acquired based on the sensed indications. Heart signals may include indications of frequency, amplitude, or other aspects of the functioning of a patient's heart. A heart rate signal may be acquired based on sensing of intrinsic electrical signals, e.g. an EKG or ECG signal. A heart rate signal may also be acquired based on sensing of pressure signals, audio signals, force signals, motion signals, or any other means for detecting at last one signal indicative of a patient's cardiac activity, alone or in combination.
Various characteristics of a heart rate signal may be processed and/or analyzed by an internal or external medical device, physician, or other user to determine one or more autonomic conditions of a patient. One example of a heart rate signal characteristic is Heart Rate Variability (HRV). HRV is a rate at which a heartbeat changes in time. HRV may have attributes including amplitude, frequency, average heartbeat rate, and coherence. HRV attributes may be indicative of functioning of a patient's autonomic nervous system. Changes in autonomic function may indicate one or more autonomic conditions of a patient. Autonomic conditions may also be related to hemodynamic conditions of the patient. Some examples of hemodynamic conditions that may be indicated by autonomic attributes include increased risk of coronary heart disease (CHD), cardiovalscular disease mortality (CVD), or a risk of arrhythmia.